Safety is a major concern when using lithium ion batteries (LIB) in hybrid electric vehicles (HEVs), pluggable HEVs and EVs. A separator that can improve the safety issues associated with LIBs and also meets assembly and cell performance requirements as well as the cost criteria is needed for the HEV applications. This invention describes and claims such an improved separator.
Currently there are two types of secondary lithium ion batteries:
1. Those with cathode containing cobalt for high energy density batteries used in cell phones, notebook PCs and consumer electronics, which require a shutdown temperature activation of 130-150° C. and melt integrity of more than 150° C.; and
2. Those with a non-cobalt cathode (mostly phosphate based) for high power batteries, which do not require a shutdown temperature capability, but must have a separator with high temperature resistance.
Lithium-ion cells have two to three times higher energy density than nickel metal hydride batteries used in the current HEVs. Due to this high energy density of lithium ion batteries, automakers are eager to replace the currently used nickel metal hydride battery packs in HEVs with a high power and high density lithium ion battery pack. Thus far, the safety issue (due to potential thermal run away of lithium ion batteries) has been a major problem, preventing the use of lithium ion batteries in the HEV applications. Among all of the commercially available polyolefin separators for LIB applications, none could pass the safety requirements for HEV applications. The only battery separator that is commercially available and has proven that it meets the safety requirements of HEVs is a ceramic separator called Seperion® from Deggusa, the international chemical company headquartered in Dusseldorf, Germany. Seperion is produced by a non-woven polyethylene terephthalate (PET) precursor impregnated on both sides with ceramics containing nano particles of Al2O3 and SiO2. Safety tests done by Deggusa, Sandia National Labs and the US Army Research Labs have proven that indeed Seperion does improve the safety problems associated with LIBs. Zhang et al. reported:
“In nail penetration test on the 8 Ah Li-ion pouch cells, it was shown that the maximum temperature of the cell using Separion separators was only 58° C. with a weight loss of 0.5% after nail penetration test, while that of the control cell using PE separators reached over 500° C. with a weight loss as high as 56.1%. Since the maximum temperature (58° C.) in the nail penetration test is far from the melting point of the PE materials, one may assume that the exceptional safety behavior of the Separion separator is more related to the nano-size ceramic materials, instead of PET non-woven matrix.” S. S. Zhang, et al. “Journal of Power Sources 164 (2007) pp. 351-364.
However, due to a complicated phase inversion manufacturing process that has been used in the production of the Separion separator, it has not been produced at a low cost, and therefore, it does not meet the cost criteria of lithium ion batteries in general and HEV/EVs in particular. In the current invention, in addition to offering comparable safety features, this invention replaces Al2O3 and SiO2 with kaolin (a low cost clay mineral filler consists of Al2O3 and SiO2), and utilizes a low cost process. The current invention does not require a nonwoven material and subsequently conversion to a microporous membrane using an expensive phase inversion method. The wet process used in the current invention has proven track records; it is simple and has been used in the production of low cost lead acid PE separators for decades.
One aspect of the current invention provides a high performance low cost ceramic-like microporous separator high in air permeability of less than 200 sec/10 cc, preferably less than 10 sec/10 cc, and with a shutdown temperature between 130-150° C. This invention also provides a method for producing the same for consumer LIB applications.
Another aspect of the current invention provides a non-shutdown polyolefin ceramic type microporous separator with high abuse tolerance but with relatively low cost that meets both the safety and cost requirements of LIBs for EV/HEV applications.
The microporous membranes of current invention will have applications in air filtration, water purification (a filter for separating microorganisms and viruses from water), size exclusion, sanitary napkins, breathable closing and house wrap.
Inert fillers are also used in the production of battery separators, primarily for achieving better pore structures (added tortousity) and increased porosity. However, fillers can also add properties such as structural integrity (high puncture resistance), reduced shrinkage, improved thermal stability, and fire retardation. They also keep the battery electrodes separated at high temperatures.
Examples of polymeric sheets with inert fillers include those described in U.S. Pat. Nos. 3,351,495, 4,287,276, and U.S. Pat. Nos. 6,372,379 and 6,949,315 (by current authors), in which, the electrolyte is capable of passing through the separator through microporous channels.
In U.S. Pat. No. 6,949,315 by the current inventors, TiO2 filler is used to improve the high temperature resistance of the separator. Addition of TiO2 to the formulation did indeed improve the thermal resistance of the separator, however, TiO2 is a heavy mineral (has a density of about 4.2 gr/cm3) and is also very expensive and not particularly affordable to be used abundantly in commercially priced separators for lithium ion batteries. Kaolin clay, in contrast, has lower density (density of 2.6 gr/cm3), is very stable in the lithium ion battery environment and is relatively inexpensive. In addition, kaolin clay has the capability to absorb significantly more oil than TiO2 (it creates more air permeability) that leads to higher ionic conductivity of the separator.
Silica has also been used as a low-cost filler in battery separator applications for decades. However for use in lithium ion battery applications silica alone, without the presence of the aluminum oxide, may not improve the high temperature performance of the separator. In addition, due to silica filler's high moisture content, it may not be suitable for lithium ion batteries.
Kaolin clay is an abundant mineral and is a common constituent of the earth's crust. Clay occurs in many different forms, but kaolin or china clay is the purest and most versatile. Kaolin clays contain Al2O3 and SiO2 with similar high heat resistance property as the ceramic material used in the Separion separator, but cost significantly less. That is why Kaolin clays are commonly used in paints, paper, plastics, rubber, ink, pigments, fiber glass, cosmetics, cement and concrete, adhesive and sealants, cable and wire. They further have advantageous properties of hardness, opacity, abrasion resistance, high brightness, and particle size. They promote flattening and easy dispersion.
Calcined Kaolin, another inert filler appropriate for use in the present invention, is an anhydrous aluminum silicate produced by heating ultrafine natural kaolin to high temperatures in a kiln. The calcination process increases whiteness and hardness, improves electrical properties, and alters the size and shape of the kaolin particles.
Kaolin clay's nominal chemical properties are generally described as follows: Silicon dioxide (wt %)=56.91, Iron oxide=0.93, Aluminum oxide=39.68, Titanium dioxide=0.54, Calcium oxide=0.16, Magnesium oxide=0.16, Sodium oxide=0.60, and Potassium oxide=0.60
Clean kaolins are calcined by firing the powder in a rotary calcining kiln to a temperature high enough to effect loss of crystal water (and accompanying mineral change). Calcined kaolin normally converts to mullite during this process. Based on where kaolin has been mined, the above chemical properties could slightly vary in the composition of their trace elements.
For both shutdown and non-shutdown separators, the current invention uses between 5% to 80% by weight kaolin, more preferably calcined kaolin as property enhancing filler (to achieve high heat resistance) in the microporous membrane's formulations. In another version of this invention, kaolin clay can be replaced with materials consisting Al2O3 and SiO2. However, Al2O3 and SiO2 may not be as economical as kaolin in this application.
Different polyolefin polymers have been used in prior arts for making battery separators used in different applications, including lead acid, alkaline and lithium ion batteries. Polymers used in the current invention are selected from ultra high molecular weight polyethylene (UHMWPE with molecular weight more than 1 million) and polypropylene (PP with a melt index of less than 2) or a mixture thereof as frame polymers and a high-density polyethylene (HDPE) having molecular weight between 300,000 to 900,000 for achieving shutdown behavior between 130-150° C. For heat resistance separators (non-shutdown), the current invention uses UHMWPE, with molecular weight more than 1 million, and PP or a mixture thereof without HDPE.
The current invention basically utilizes a commonly used prior art method widely used for producing battery separators for lead acid, alkaline and lithium ion cells. This process starts by mixing and extruding polymers, filler (in this case, kaolin, calcined kaolin or a mixture of Al2O3 and SiO2), with a plasticizer (oil) at high temperatures and pressure through a film die, casting the sheet, and wet stretching, either uni-axial or biaxial. Followed the wet stretching the oil is removed by solvent extraction and heat setting, creating a microporous sheet. To achieve higher air permeability, the stretching should be done after the extraction step.